This invention relates to the field of telephony, and in particular to a method and apparatus for connecting broadband voice and data signals to telephone systems. The invention is suitable for, but not limited to, interfacing a DSL analog front end circuit to a subscriber line in the presence of a voice band (DC-4000 Hz) signal. DSL (Digital Subscriber Line) is a service that permits broadband data signals to be superimposed on local subscriber loops.
With the popularity of the Internet, telephone companies are offering more and more broadband services to subscribers. One such service is DSL (Digital Subscriber Line) wherein the voice band Plain Old Telephone Service (POTS) and the higher frequency band DSL service are put onto the same pair of copper wires at the same time. However, this gives rise to several problems. Both the DC and AC requirements for the voice band POTS and the DSL service are different. POTS requires a DC voltage and current for line signaling and voice transmission while DSL service only operates in AC mode. The line impedance for POTS lines ranges from 600 to 1000 ohms either real or complex, while the DSL line impedance is around 100 ohms.
When POTS is in the on-hook state, no voice (AC) signal is transmitted onto the tip/ring pair. There is about 40V to 48V DC voltage across tip and ring. Usually, the tip DC voltage is around 0V to xe2x88x925V and the ring DC voltage is around xe2x88x9240V to xe2x88x9256V. A typical DSL signal has a peak voltage of about 18V. When the DSL signal is transmitted during the POTS on-hook state, the typical signal at tip and ring is as shown in FIG. 2.
There are currently two common approaches for providing both POTS and DSL service on the same tip/ring pair. One approach is to use a transformer and a splitter. The splitter consists of two capacitors to the DSL interface connection and a low pass filter (LPF) for the POTS connection as shown in FIG. 3.
The two capacitors act as a high pass filter and isolate the DSL line impedance from the voice band POTS line. The capacitors also isolate the DC voltage for POTS line from the DSL line driver. The drawback of this method is the need for the relatively expensive and bulky transformer and splitter.
The other approach is to use common solid state differential drivers to directly drive both POTS signals and DSL signal onto the common copper wire pair as illustrated in FIG. 4. The total feeding impedance, 2Zf, is set to that of the DSL load impedance. As shown in FIG. 2, the DSL signal requires an AC swing of about 40Vpp. POTS requires a DC voltage of about 40V to 48V between tip and ring when the line is in onhook (idle) state to meet Central Office (CO) requirements. In the case of DSL signal transmission during POTS on-hook state, the drivers need a DC supply voltage of about 80V (V+minus Vxe2x88x92equals to 80V) to drive signals onto tip and ring as illustrated in FIG. 2.
For a typical DSL load of 100 ohms and a signal of 3Vrms, the drivers need to provide about 30 mA. Hence the power consumption is around 2.4 W for a 80V power source. In this configuration, about 50% of the power is wasted in the line drivers due to the fact that they need to be biased to 80V instead of 40V to provide the required tip/ring DC voltage. The power dissipated through the DSL load and the feeding component is about 0.2 W, (2xc3x97(100 ohmsxc3x9730 mA2)). Hence the power dissipation in the drivers is about 2.2 W. For normal integrated voice and data telephone line service, the majority of the time the POTS line is in on-hook state while DSL line is active. It makes the common solid state driver very power inefficient.
An object of the invention is to alleviate this problem.
According to the present invention there is provided an apparatus for supplying a tip and ring telephone line with voice band and broadband signals, comprising a first pair of drivers for supplying at least DC signals to said respective tip and ring line components, a second pair of drivers for supplying broadband signals to said tip and ring line components, and feed components for combining the outputs of said respective drivers for said respective tip and ring components of the telephone line.
The broadband signals are typically DSL signals. The voice band signals can either be passed through the first pair of drivers with the DC signals or the second pair of drivers with the broadband signals.
In one embodiment the feed components match the output impedance to the line impedance in the frequency band of the broadband signals, whereas the output impedance in the voice band is matched by feedback of the voice band transmit signal.
The drivers are typically solid state integrated circuits devices.
The invention also provides a method of improving power efficiency in the transmission of integrated voice and broadband service on a tip and ring telephone, comprising supplying at least DC signals through a first pair of drivers for to said respective tip and ring line components, supplying broadband signals to a second pair of drivers for said tip and ring line components, and combining the outputs of said respective drivers with feed components for said respective tip and ring components of the telephone line.